1. Field of the Invention
The present invention relates to a recording-medium driving apparatus, and more particularly, to a disc apparatus that drives a disc-shaped recording medium.
2. Description of the Related Art
In recording-medium driving apparatuses, access to a recording medium is performed in a contact manner or a noncontact manner in order to read data recorded on the recording medium or to write new data onto the recording medium. For example, when the recording medium is an optical disc, an optical head having a lens accesses the optical disc that is clamped by a spindle motor.
In such recording-medium driving apparatuses using an optical head, the gap between the recording medium and an objective lens for data reading and writing tends to decrease with increases in capacity and density. When the recording medium is a CD (compact disc), laser light passes through a substrate having a thickness of 1.2 mm. When the recording medium is a DVD (digital versatile disk) serving as a bonded medium, laser light passes through a substrate having a thickness of 0.6 mm. In a Blu-ray Disc, laser light reaches a recording layer through a protective layer having a thickness of 0.1 mm, and the gap between the disc and the objective lens is of the order of hundreds of micrometers.
In order to achieve large capacity and high density, near-field technology using near-field light and an objective lens called a solid immersion lens (SIL) has been proposed. A recording-medium driving apparatus using near-field technology uses the following property of laser light. That is, in a case in which the refractive index n and the numerical aperture (NA) of the objective lens are increased, all incident laser light is returned (totally reflected) when the gap between the objective lens and a recording medium is large, and a slight amount of laser light leaks when the gap is small. The objective lens including an SIL has a numerical aperture NA of 1 or more, and a density of 80 gigabyte/inch is achieved by this technology.
In this recording-medium recording apparatus using an optical head, a removable recording medium is clamped by a spindle motor, and vertical or radial deviation of the clamped recording medium sometimes causes a problem. There are various causes of deviation, for example, vertical movement of a shaft of the spindle motor, working accuracy of a clamp hub of the spindle motor on which the recording medium is placed, and warping of the recording medium. It is also conceivable that deviation is caused by displacement of the recording medium due to adhesion of dust present in the air onto the clamp hub.
Even when a large deviation of ±40 μm occurs between a recording medium having a relatively small capacity, such as a CD or a DVD, and the shaft, the influence of deviation can be absorbed by a focusing servo provided in an actuator including the objective lens. However, in near-field technology, the gap between the recording medium and the objective lens is set to be approximately 100 nanometers or less in order to avoid total reflection. Moreover, in order to read signals, it is necessary to maintain an extremely small gap of at least tens of nanometers. Therefore, the recording medium and the objective lens may collide with each other even when the gap therebetween slightly varies. In this way, vertical or radial deviation is influential particularly in the near-field technology.
In contrast, in a hard disk apparatus using magnetic recording, a small gap of 10 to 20 nanometers is provided between a magnetic head and a recording medium. However, collision between the magnetic head and the recording medium is avoided by employing a flying head that flies because of a flying force produced by the rotation of the recording medium. Further, Japanese Examined Patent Application Publication No. 64-4276 discloses, in FIG. 3, a hard disk apparatus that includes an inner filter for removing the influence of dust in the air.